StumbleUpon is a social networking site for sharing websites. A recent informal experiment conducted by the Social Media Optimization blog found that submitting pages to StumbleUpon drove the most traffic to those pages, and Darren Rouse on Problogger found similar results when analyzing web traffic to a photography site.  StumbleUpon works as follows: Users select topics of interest, and use the StumbleUpon browser toolbar to submit pages they like during the course of their normal Web browsing. Users can click the Stumble! button in the toolbar to browse sites StumbleUpon has determined they might like based on their interests, the pages they've submitted, and the pages other users with similar interests have submitted. StumbleUpon also has social networking features enabling users to add other users as friends, and join interest-based groups. The pages that your friends and your groups have selected as favorites are included in the sites StumbleUpon recommends to you. Users can browse other user's profile pages, interests, and favorite websites, and can send website recommendations directly to other users.

The following are 3 tips for using StumbleUpon to drive more traffic to your own website, posted to the Blog Design, Marketing, Advertising, SMO, SEO and Keywords Discussion blog

1) When adding categories and tags, select the most fitting Topic from the most popular categories, then, starting with the most popular tags that are applicable to your submission, add increasingly specific ones.

2) Use the "friends" feature. Add people who vote for your submissions as friends, and, if you are comfortable doing so, send them a message asking them to add you back. Browse other users' profiles to find people who have interests similar to yours, and add them as friends. To increase the chance that they will friend you back, look for users who have a low to moderate number of friends (50-150).

3) Use the "send to" feature to send things to particular friends whom you think will be especially interested in your site.